output of following c code

[shivakumar]

#include<stdio.h>
void main()
{
int a;
printf("a contains %d\n");
}

this is giving some junk value where as

#include<stdio.h>
void main()
{
float a;
printf("a contains %f\n");
}

this is not giving any value.I am not getting why is it so and could you please tell me what is the concept behind it?